波流联合作用深吃水半潜平台涡激运动数值研究

The vortex-induced motions (VIM) of deep-draft semi-submersible in combined wave and current can be characterized as the high-Reynolds-number flow, free surface large deformation, large amplitude six-degrees-of-freedom (6DOF) motions, floating and mooring, etc. For VIM in pure current, the representative numerical approach is viscous computational fluid dynamics (CFD) consist of detached-eddy simulation (DES), 6DOF rigid body motion and deforming grid, horizontal equivalent spring model. However, to account for the combined wave and current effect on VIM, there are several barriers such as the wave damping for high-Reynolds-number flow and the mesh motion constrain of deforming grid. To resolve these issues, a buoyancy-modified turbulence model is proposed, 6DOF rigid body motion and dynamic overset grid technique is combined in the present project. An high efficiency, accuracy and robust analysis tool for predicting VIM in combined wave and current is developed. Numerical experiments are carried out to validate the accuracy of the developed solver. A series numerical results are provided as standard numerical benchmark cases. The outcome of this project are expected to offer an analysis tool for the optimal design and safety operation of deep sea floating offshore structures.

波流联合作用的深吃水半潜式平台涡激运动具有高雷诺数三维流动、自由面大变形、平台大幅度六自由度运动、漂浮及系泊等特点。目前对纯流作用的涡激运动问题已经形成了一套以分离涡模拟、六自由度运动及变形网格、水平等效弹簧系统等数值方法为核心的计算流体力学研究手段。然而由于高雷诺数湍流模拟中的波浪衰减、动网格对物体大幅度运动的限制等诸多因素，这些已有的数值方法还无法模拟波流联合作用下平台涡激运动问题。针对以上问题，本项目重点研究考虑浮力修正的两相流湍流模拟、六自由度运动边界与动态重叠网格更新匹配技术，研发一套效率高、精度和稳定性好的波流联合作用半潜式平台涡激运动计算程序，通过标模算例进行数值方法验证，形成可供参考波流联合作用涡激运动标准数据库。研究成果可为深海浮式结构物的优化设计和安全运营提供计算分析工具。

涡激运动是由涡激力引起的平台大幅度周期性往复运动，对立管和系泊系统的疲劳损害有重大影响。本项目建立了高精度自由面流动求解模型：在非结构网格框架下开发了考虑自由面压力跳跃条件的虚拟流体方法，在求解压力梯度时用虚拟流体网格替换实际网格的压力，解决了自由界面附近动压扩散的问题；研究了考虑自由面湍流粘度浮力修正的湍流模型，改善了波流耦合情况下高雷诺数自由面流动问题的波浪衰减问题；研究了Liutex/Omega-Liutex等第三代涡识别方法，并开发了相应的计算程序，解决了自由面流动问题中涡结构识别和提取不准的问题。研究了四立柱半潜式平台的涡激运动及其动力学响应特性，揭示了不同来流角度、不同折合速度对四立柱半潜式平台涡激运动特征的影响机理和规律。本研究成果可为我国深海油气开采的浮式平台设计提供有力工具，对深海浮式平台的设计和安全运行具有重要意义。